Angel et al. [1]} noted that the \(2N\)  non-trivial eigenvalues of the \(2L\times 2L\)  non-back tracking matrix \(B\)  from (REF ) are contained in eigenvalues of the \(2N\times 2N\)  matrix \(B^{*}\) :
\(B^{*} = \begin{bmatrix}A & I - \Delta \\I & O\end{bmatrix},\) 
